To elaborate, it doesn't make much sense in my opinion to give them native bonuses to Hell terrain, as they are still human and leaving in Erebus at Game Start ( Contrary to the Infernals who are Demons), but them gaining it through rituals make sense and make for good gameplay, i think.
Nice idea!, make it so that they gradually trade off their 'human' bonus for their 'demonic' equivalent with every rituals completed. Kinda like the Sheaim slowly losing their souls and human moralities.
Actually, the ritual system i think is one of the best underused system. And almost all the good one were preserved for the Illians. Adding some more for the Sheaim, the Amurites, other arcane-centric nations or even rituals for any nations would be a nice addition.
Some of my idea for low level rituals. These are Rituals that requires little production to complete but also has low effect compared to other 'game changing' rituals.
-Mass Bless(name pending): Unlocked at Priesthood. cost 200

Double production speed with Law mana and Sun mana. Can be cast multiple times.
Add Blessed promotion to 10 random legible units in the caster's civ. It's a kinda nifty ritual when preparing for the apocalypse. Plus, the Blessed promotion will be remove after combat so i think it doesn't seems to overpowered much.
-Mass Regeneration(name pending): Unlocked at Alteration. cost 200

Double production speed with Body mana. Can be cast multiple times
Add Regeneration promotion to 10 random wounded units in the caster's civ.
-Sooth the souls: Unlocked at Knowledge of the Ether. cost 300

Double production speed with Mind mana and Spirit mana. Can be cast multiple times.
Remove all negative mind effect(crazed, enraged etc.) from 2 random afflicted units per ritual. I absolutely despised these uncurable mental sickness especially at the start of the game. I know dungeon delving was supposed to be a gamble but atleast give us some way to remove these malus, even if we have to waste alot of time completing the ritual to cure only one or two units.
-Nature's bounties: Unlocked at Animal Handling. cost 400

Double production speed with Life mana and Nature mana. Can be cast multiple times.
Greatly increase animals spawn rate and instantly spawn more animal lairs on the map for 30 turns. Stack with Double animals spawn option. I loves animal in FFH and wished there was more ways to play with them or find some way to use them to affect or deter our rivals.
-Natural Cycle: Unlocked at Animal Handling. cost 300

Double production speed with Life mana and Nature mana. Can only be cast while Nature's bounties is active.
Revert animals spawn rate from Nature's Bounties back to normal
Oh and can i get a confirm again about Acheron the Red Dragon, whether she was able to move or not? I vaguely remembered the answer in this thread that she's supposed to be leashed and cannot move at all. But in my hot seat session with my friend Acheron has about 5 leash range and decimate my friend's axemen far away from the dragon's city. This time i didn't use any outdated modules.